Add dependabot to upgrade GitHub Actions (qiskit-community#1373)

I noticed some of our GitHub Actions, like checkout@v3, are outdated.
Dependabot should be able to check these automatically and create pull
requests. I didn't add `requirements.txt` to Dependabot since we don't
have that many dependencies and we usually only bump the minimum version
if needed.

.github/dependabot.yml

@@ -0,0 +1,7 @@
+# Check for updates to GitHub Actions
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "daily"